สอบถามเกี่ยวกับ Visual Studio 2010 ครับ, build project สำเร็จ แต่ตอนเปิด application แล้ว error ครับ

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Data.SqlClient;

namespace WindowsFormsApplication18
{
 public partial class Form1 : Form
  {
    public Form1()
    {
        InitializeComponent();
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        string constr = @"Data Source=.\SQLEXPRESS;
                        AttachDbFilename=|DataDirectory|\PUBS.mdf;
                        Integrated Security=True;User Instance=True;";
        SqlConnection connection = new SqlConnection(constr);
        connection.Open();
        string sql = "SELECT * FROM authors";
        SqlCommand command = new SqlCommand(sql, connection);
        textBox1.Text = (string)command.ExecuteScalar();
    }

  }
}